PCIe transmitters “detect” the presences of its link partner's receiver as part of the protocol. O/E (Optical/Electrical) interfaces do not provide the correct termination on the electrical side of the interface, and do not provide a mechanism over the optical side of the cable. This interferes with the PCIe protocol. This presents a technical problem for which a technical solution using a technical means is needed.
PCIe uses a differential signaling protocol. Differential signals below 65 mV are defined as Electrical Idle. Electrical Idle is used as part of the PCIe protocol. Optical cable is a single ended interface. When electrical to optical devices are presented with electrical Idle (i.e. a differential signal below 65 mV) random noise is detected and sent over the cable. This interferes with the PCIe protocol. This presents a technical problem for which a technical solution using a technical means is needed.
Optical/Electrical interfaces behave like a simple repeater, i.e. have a fixed de-emphasis on the electrical transmitter. This interferes with the 8.0 GT/s link equalization procedure. This presents a technical problem for which a technical solution using a technical means is needed.
For the CEM specification there are side band signals to support hot plug, PERTSN etc. The side band signals are not supported over an optical interface. This presents a technical problem for which a technical solution using a technical means is needed.